Aggregates are essential ingredients of concrete, masonry, and cavity fill insulation. Other applications for aggregates include filler aid or horticultural aggregate. Aggregates may be derived from natural sources with minimal processing or from naturally occurring materials that are heat treated. Aggregates may also be synthetic. Aggregates from natural sources, such as quarries, pits in ground, and riverbeds, for example, are generally composed of rock fragments, gravel, stone, and sand, which may be crushed, washed, and sized for use, as needed. Natural materials that may be used to form aggregates include clay, shale, and slate, which are pyroprocessed, causing expansion of the material. OPTIROC and LECA are examples of commercially available expanded clay aggregates, for example. Synthetic aggregates may comprise industrial byproducts, which may be waste materials. LYTAG, for example, is a commercially available sintered aggregate comprising pulverized fuel ash (“PFA”), also known as fly ash. PFA is produced from the combustion of coal in power plants, for example.
Natural aggregates for use in construction are in very high demand. However, aggregate resources are finite and extracting and processing these materials is complicated by environmental issues, legal issues, availability, urban expansion, and transportation costs, for example. There has also been a tremendous increase in waste generation by various industries that must be disposed of in an environmentally and legally acceptable manner. Typically, most generated waste is disposed of in landfills at a great expense. Due to the exhaustion of available landfill sites, the difficulties in acquiring new sites, the potential adverse environmental effects, and the cost of landfilling, disposal of waste materials has been a significant problem for many years.
The processing and transformation of waste materials to produce viable synthetic aggregates for use in concrete and other applications would alleviate both waste problems and the depletion of natural aggregate resources.
Aggregates may be lightweight or normal weight. Lightweight aggregates (“LWAs”) have a particle density of less than 2.0 g/cm3 or a dry loose bulk density of less than 1.1 g/cm3, as defined in ASTM specification C330. Normal weight aggregates from gravel, sand, and crushed stone, for example, generally have bulk specific gravities of from about 2.4 to about 2.9 (both oven-dry and saturated-surface-dry), and bulk densities of up to about 1.7 g/cm3. High quality LWAs have a strong but low density porous sintered ceramic core of uniform structural strength and a dense, continuous, relatively impermeable surface layer to inhibit water absorption. They are physically stable, durable, and environmentally inert. For use in concrete, LWAs should have a sufficient crushing strength and resistance to fragmentation so that the resulting concrete has a strength of greater than 10 MPa and a dry density in a range of about 1.5 g/cm3 to about 2.0 g/cm3. Lower density LWAs may also be produced. Concrete containing LWAs (“LWA concrete”) may also have a density as low as about 0.3 g/cm3.
Synthetic lightweight aggregates (“LWAs”) have received great attention due to the substantial benefits associated with their use in structural applications. Concrete containing LWAs may be 20-30% lighter than conventional concrete, but just as strong. Even when it is not as strong as conventional concrete, the LWA concrete may have reduced structural dead loads enabling the use of longer spans, narrower cross-sections, and reduced reinforcement in structures. The lower weight of the LWA concrete facilitates handling and reduces transport, equipment, and manpower costs. LWA concrete may also have improved insulating properties, freeze-thaw performance, fire resistance, and sound reduction.
Sewage sludge, which is produced by biological wastewater treatment plants, is a significant waste in terms of volume and heavy metal content. Sewage sludge comprises settled solids accumulated and subsequently separated from the liquid stream during various treatment stages in a plant, such as primary or secondary settling, aerobic or anaerobic digestion or other processes. The composition and characteristics of the sewage sludge may also vary depending upon the wastewater treatment process and the sewage sludge treatment process applied. Sewage sludge can be raw, digested, or de-watered. Sewage sludge contains significant amounts of organic materials and may also contain high concentrations of heavy metals and pathogens. Sewage sludge has generally been disposed of by incineration to form an inert ash that is disposed by lagooning, landfilling, spreading on land as fertilizer or soil conditioning, and ocean dumping, for example. If the sewage sludge has not been treated prior to being spread on land or disposed of in a landfill, undesirable contamination may occur.
Sewage sludge recycling and disposal presents considerable economic and environmental problems. The presence of heavy metals and pathogens in the waste, which may leach from the landfill, is a threat to adjacent ground and water supplies. The availability of landfill sites is also decreasing. In addition, the presence of large amounts of water in sewage sludge, which increases the weight of the waste, causes significant transportation and disposal costs.
Another significant waste produced is the ash stream generated by municipal solid waste (“MSW”) incineration. Although the disposal of MSW ash residues to landfill occupies only one-tenth of the volume of the original waste, their management presents a problem due to considerable amounts of solid residues produced, the majority of which is currently landfilled. Incinerator bottom ash (“IBA”) is the principal ash stream accounting for approximately 75 to 80% of the total weight of MSW incinerator residues and is a heterogeneous mixture of slag, glass, ceramics, ferrous and nonferrous metals, minerals, other non-combustibles, and unburnt organic matter. IBA is currently used in its raw form (without heat treatment) in the construction of embankments, pavement base and road sub-base courses, soil stabilization, bricks, blocks, and paving stones, and as fillers in particular applications. Although considered a relatively inert waste, leaching of heavy metals in these applications is possible.
MSW incineration also produces a particulate residue in the form of dust suspended in the combustion gases or collected in emission control devices, which is called air pollution control (“APC”) residue. This includes fly ash, lime, carbon, and residues collected at the pollution control systems. The incinerator filter dusts (“IFD”) are an APC residue collected in baghouse filters produced at a rate of 25-30 kg per 1000 kg of incinerated waste, while fly ash, which in some cases includes IFD, accounts for about 10% to 15% of the total combined ash stream. MSW incinerator fly ash (“IFA”) contains high concentrations of hazardous materials, such as heavy metals, dioxins, sulphur compounds, and chlorine compounds, and is therefore classified in most European countries as a toxic and dangerous residue. Therefore, it can only be disposed of in special landfills, which is costly and environmentally unsafe.
Significant volumes of residues are also produced by the mining of minerals, ores, and stones. Typical mining operations include extraction, beneficiation, blasting, crushing, washing, screening, cutting (stone), and stockpiling. These operations produce wastes, such as crushed material of different sizes, powders, mud residues, and waste water, that must be disposed of. Marble and granite rejects, from cutting ornamental stones, for example, include large amounts of rejected mud that is discarded into rivers and lagoons. Granite sawmills and granite cutting machines also generate large amounts of powder and mud waste residues. The term “mining waste” is used herein to refer to the waste produced during these operations. Mining waste needs to be treated prior to lagoon or landfill disposal in order to prevent environmental contamination. Other mining wastes include limestone and dolomite tailings, for example.
Electricity-generating power plants also produce large volumes of ash residues in the form of a fine-grained particulate, known as pulverized fuel ash (“PFA”) and a coarse fraction, known as furnace bottom ash (“FBA”). The heavier ash material accounts for 20-30% of the total coal ash produced and is the fraction that falls through the bottom of the furnace. FBA is currently used in its raw form as aggregate in lightweight concrete, in Portland cement production and other asphalt or road base applications.
Other waste generated at high rates include cement kiln dusts (“CKD”) and blast furnace slag (“BFS”). CKD is a fine-powdery byproduct of cement manufacture operations captured in the air pollution control dust collection systems of the manufacturing plant. Approximately 14.2 million tons of CKD are produced annually in the United States, and about 64% of the total CKD generated is reused within cement plants. BFS is a nonmetallic product of the production and processing of iron in blast furnaces. It is estimated that approximately 15.5 million tons of BFS are produced annually in the United States, and the majority is used in cement production, as an aggregate or insulating material.